The amount of cross-sectional area narrowing in each 5-mm long segment of each of the 4 major epicardial coronary arteries was determined in each of patients with type III hyperlipoproteinemia and symptomatic, fatal Atherosclerotic coronary heart disease. Four had had angina pectoris; 2; acute myocardial infarcts which healed, and 2 died suddenly. Of the 4 major epicardial coronary arteries, all 4 were narrowed 76-100% in cross- sectional area by atherosclerotic plaques in 2 patients, 3 were narrowed to this degree in 2 patients and 2 were so narrowed in 1 patient. Three patients had severe narrowing of the left main coronary artery. The percent of 5-mm long segments of coronary artery narrowed of various degrees was as follows: 96-100%, 0-37 (mean 14); 76-95%, 14-61 (mean 35); 51-75%, 9-41 (mean 24); 26-50%, 0-42 (mean 16), and 0-25%, 0-27 (mean 11). Utilizing a scoring system of 1 to 4 for the 4 categories of narrowing (1=0-25%, 2=26-50%, 3=51-75% and 4=76-100% cross-sectional area narrowing), the scores per 5-mm segment for each patient ranged from 2.5 - 3.9 (mean 3.1).